This invention relates to a clamping circuit for color television signals and, more particularly, to an improved clamping circuit which clamps the red, green and blue color signals to the pedestal level, which improved circuit is far less complicated and less expensive than clamping circuits heretofore known.
A conventional color television signal, such as the NTSC signal, is formed of successive line intervals of video information, each line interval including a horizontal synchronizing pulse, a burst signal, a luminance component and a chrominance component. Typically, the horizontal synchronizing pulse is a negative-going pulse which is referenced to a pedestal level. This pedestal level also provides a reference level upon which the burst signal is superimposed. Also, in order to determine the luminance and chrominance information, the levels of the luminance and chrominance components generally are referenced to the pedestal level. Thus, it is appreciated that, in order to properly demodulate a color television signal and to display an accurate video picture, it is necessary that the pedestal level of the received color television signal be detected correctly. However, during the transmission of a color television signal, the pedestal level thereof, which may be considered a d.c. reference level, often fluctuates from its proper level. Accordingly, in a conventional television receiver, a clamp circuit must be provided to detect such fluctuations in the pedestal level and to correct or compensate for such fluctuations, thereby clamping the color television signal to the proper pedestal level from which the various horizontal synchronizing pulses, burst signals, luminance component and chrominance component can be measured and detected.
Since the luminance and chrominance components of a composite color television signal are processed by separate channels in a color television receiver, it is quite likely that the pedestal level of the chrominance component may vary in a manner which differs from the variations in the pedestal level of the luminance component. Therefore, it is desirable to provide a clamp circuit in the chrominance channel so that the chrominance component can be clamped correctly to its proper pedestal level. This clamp circuit is formed of three separate and independent clamping circuits which are used for the red, green and blue color difference signals, respectively. Each clamping circuit includes a comparator for comparing a respective color difference signal to a reference level, such as the desired pedestal level, and any difference therebetween is sampled at the time of occurrence of the pedestal level in the color television signal. This difference between the actual and desired pedestal levels is stored and added to the color difference signal so as to adjust its pedestal level to be equal to the desired pedestal level. However, since three separate and independent clamping circuits must be used to clamp the red, green and blue color difference signals, this arrangement is relatively complex and expensive.
In another proposal of a clamp circuit for use in the chrominance channel of a color television receiver, the complexity of the circuit is reduced by simplifying the clamping circuit which is used in conjunction with the green color difference signal. In this proposal, the red and blue color difference signals are provided with separate clamping circuits, as before. A matrix circuit is connected to receive the clamped red and blue color difference signals and to produce the green color difference signal therefrom. It is assumed that, since the red and blue color difference signals are clamped to the proper pedestal level, then the green color difference signal which is derived therefrom, that is, which is produced by the matrix circuit, likewise will be clamped to the proper pedestal level, thus obviating the need for the comparator and sampling circuit which previously had been used with the green color difference signal. However, a typical matrix circuit is a resistor matrix. As is known, the resistance values of a resistor matrix generally are within allowable tolerances and, moreover, may drift over a period of time. This means that the pedestal level of the green color difference signal may not be equal to the proper pedestal level even though the red and blue color difference signals are properly clamped. As a consequence thereof, when the green color difference signal is combined, or matrixed, with the luminance component, the green drive signal which is obtained will have a fluctuating pedestal level.